1. Field of the Invention
The present invention relates to a radar transponder serving as a lifesaving device for rescue activities.
2. Description of the Background Art
A radar transponder using electromagnetic waves of a radar frequency band (hereinafter referred to as xe2x80x9cradar electric wavexe2x80x9d) has been hitherto known as a lifesaving device. The radar transponder serves to receive a radar electric wave emitted from a search radar installed in an air plane or the like, and then emits a response electric wave in response to the radar electric wave thus received. More specifically, the search radar emits the radar electric wave every predetermined emission period, and the emission period is set to 1 msec, for example. The above or another search radar receives the response electric wave from the radar transponder until it emits a next radar electric wave, whereby the search radar can known the locating position of the radar transponder.
Triggered by the reception of the radar electric wave transmitted from the search radar, the radar transponder generates a predetermined number of (for example, twelve) sweep signals. FIG. 11 is a waveform diagram showing the sweep signals generated in the conventional transponder.
The sweep time xcex94T of each sweep signal is set to 8 xcexcsec, for example. Accordingly, the time xcexa3xcex94t needed to generate the twelve sweep signals is equal to 8xc3x9712=96 xcexcsec. After generating the twelve sweep signals, the radar transponder does not generate any sweep signal until it receives the next radar electric wave. When receiving the next radar electric wave, the radar transponder generates a predetermined number of sweep signals again on the basis of the reception of the next radar electric wave as a trigger. The radar transponder repetitively executes the above processing to detect the radar transponder.
The search radar which emits the radar electric wave can basically receive the response electric wave from the radar transponder. However, there is a case where even a search radar which emitted a radar electric wave cannot receive a response electric wave thereto due to surrounding environments of the radar transponder. More specifically, for example when a radar transponder is mounted on a ship sailing on the sea, the passage of the response electric wave is intercepted by waves and thus the response electric wave hardly reaches the search radar. Further, the response electric wave emitted from the radar transponder is reflected from the surface of the sea and it becomes a multipass wave. Accordingly, even when the search radar is located near to the radar transponder, it could not capture the response electric wave if the electric field intensity of the response electric wave is small. In this case, if the reception sensitivity of the search radar is low, it is more difficult to capture the response electric wave.
In order to avoid these disadvantages, it may be considered that a radar transponder is searched by using a plurality of search radars. In this case, even when a search radar emitting a radar electric wave cannot receive the response electric wave, another search radar which does not emit the radar electric wave may be located at a position where the passage of the response electric wave is not intercepted, or the electric field intensity of the response electric wave is large. Accordingly, in such a situation, it apparently seems to be possible that another search radar which does not emit the radar electric wave can receive the response electric wave.
However, even when any search radar which does not emit the radar electric wave is located at such a position, it is not necessarily possible for the search radar to receive the response electric field because the probability that the search radar can receive the response electric wave is very small.
More specifically, the search radar periodically scans the reception beam over 360 degrees because the incoming direction of the response electric wave is unclear. Further, in order to limit the existence direction of the radar transponder to a relatively narrow range, the beam width of the reception beam is set to an extremely narrow value, for example, 3 degrees. Accordingly, in this case, the probability P1 that the reception beam is directed to the existence direction of the radar transponder is equal to 3 degrees/360 degrees=about 0.008. In other words, the probability that the response electric wave transmitted from the radar transponder can be received is equal to 0.008.
The time period for which the radar transponder can emit the response electric wave is limited to an extremely small time after the radar transponder receives the radar electric wave. That is, the emission time of the response electric wave is equal to 8xc3x9712=96 xcexcsec if the radar transponder generates twelve sweep signals like the case described above. Accordingly, taking into consideration that the emission period of the radar electric wave is equal to 1 msec, the probability P2 that the response electric wave can be received during 1 msec is equal to 96 xcexcsec/1000 xcexcsec=0.096.
From the above result, the probability P that the search radar can receive the response electric wave is equal to P1xc3x97P2=0.008xc3x970.096=0.000768. Therefore, even when one radar transponder is searched by plural search radars, there occurs a problem that the search activities are obstructed.
Therefore, an object of the present invention is to solve the problems of the above technique, and provide a radar transponder which can receive a response electric wave with high probability by a search radar.
In order to attain the above object, a radar transponder according to the present invention is characterized by including a trigger signal output unit for outputting a trigger signal in response to a predetermined trigger generating condition, and a transmitter for continuously generating plural sweep signals in response to the output of the trigger signal from the trigger signal output unit until a stop condition based on an external factor is satisfied, and transmitting a response electric wave corresponding to the sweep signals thus generated.
According to the radar transponder thus constructed, the sweep signals are continuously generated in response to the output of the trigger signal until the stop condition based on the external factor is satisfied, and thus the response electric wave can be emitted for a longer time period as compared with the conventional radar transponder. Accordingly, the radar transponder concerned can be detected with a higher probability by a search radar which searches the radar transponder. Therefore, the radar transponder according to the present invention can more effectively support lifesaving, etc.
As the stop conditions based on the external factor may be considered a stop operation of power supply to the transmitter, a manual instruction to stop the emission of the response electric wave, etc. Further, as the trigger generating conditions may be considered a reception operation of a radar electric wave emitted from a search radar, a manual instruction to emit a response electric wave, etc., detection of a predetermined fixed amount or more seawater, detection of a predetermined fixed amount or more impact, etc.